1. Field of the Invention
The present invention relates generally to fabrication of electronic components and particularly to lapping of gold contacts on wafers.
2. Description of the Prior Art
In the fabrication of integrated circuits, a semiconductor wafer typically requires numerous processing steps, including deposition, patterning, and etching. It is often desirable to achieve a pre-determined level of surface planarity and to minimize surface irregularities such as scratches and pits. In the formation of gold pads, to which electrical leads will be connected, such surface irregularities may affect the performance of the final device and/or create problems during subsequent processing steps.
Consequently, a typical process includes treatment of surface defects on the wafer surface by using a polishing pad in conjunction with slurry containing loose abrasive particles dispersed in a liquid. A problem with use of such slurries, however, is that the slurries generate a large number of particles from the surface being polished which must be removed and disposed of following wafer treatment. Additionally, the particles of slurry material, such as alumina, can become embedded in the gold pads during the polishing process. These particles can interfere with the solder-ball bonding of the electrical leads, and thus the overall performance can be seriously degraded. A second cleaning process may be required to rid the gold pad surfaces of these residual particles, which of course increases processing time, and reduces efficiency.
Additionally, during testing of electrical components, it is necessary to attach probe wires to the gold pads to supply power to various circuits or to make test measurements, or to connect them with other circuits or testing apparatus. Gold is known to be a very soft metal, and it is not unusual for the surface of the gold pads to become dented or gouged during the course of the testing. This of course disturbs the planarity of the pads and can cause irregularities in the surface finish which can degrade the contacts made with connections in the finished product. Gouging can lead to shorting between probe wires. This is shown in FIGS. 1A-1C (prior art).
In FIG. 1A, a gold pad 2 which has not been hardened includes an upper surface 4, which is approached by a contact probe 6. In FIG. 1B, the probe 6 contacts the upper surface 4, and presses into the upper surface due to the softness of the gold pad 2. In FIG. 1C, the contact probe 6 is removed, leaving a gouge 8 or dent in the upper surface 4 of the gold pad 2.
It is of course undesirable for the testing process to degrade performance of the completed component. It is therefore desirable to harden the surface of the gold contacts before connecting them with testing probes, so that the surface is not so easily damaged.
Thus there is a need for a method of lapping gold pads which does not involve the use of alumina slurry, and which hardens the surface of the gold pads to prevent damage.